Electron-rich metal centres

A number of transition-metal complexes of RNSO ligands have been structurally characterized. Three bonding modes, r(A,5), o-(5)-trigonal and o (5 )-pyramidal, have been observed (Scheme 9.1). Side-on (N,S) coordination is favoured by electron-rich (et or j °) metal centers, while the ff(S)-trigonal mode is preferred for less electron-rich metal centres (or those with competitive strong r-acid co-ligands). As expected ti (N,S)... 

In examples 2.22 a and b the metals increase their valence by two, and this is not just a formalism as indeed the titanium(II) and the nickel(O) are very electron rich metal centres. During the reaction a flow of electrons takes place from the metal to the organic fragments, which end up as anions. In these two reactions the metal provides two electrons for the process as in oxidative addition reactions. The difference between cycloaddition and oxidative addition is that during oxidative addition a bond in the adding molecule is being broken, whereas in cycloaddition reactions fragments are combined. 

Most likely the cobalt catalyst is HCo(CO)2(L), which has a very electron rich metal centre and dissociation of CO does not occur under the reaction conditions. The first step is a reaction of the cobalt hydride with ethylene oxide forming a hydroxyethylcobalt species, which does not require dissociation of... 

CO Subsequently a migratory insertion will take place. Oxidative addition of H2 will be faster at the electron rich metal centre and thus the aldehyde will form. Hydrogenation takes place at ruthenium (added as Ru3(CO)i2) as indeed catalyst systems containing cobalt only are known to give 3-hydroxypropanal as the product. 

Mo(S2CNEt2)2(CO)L2] (L = PMe2Ph or PMePh2 = dppe) have also been synthesized.62 These monocarbonyl compounds are notable for their very low value of v(CO) (1760-1735 cm"1) implying a very electron-rich metal centre in these complexes.62 They transform readily into the dicarbonyl complexes with CO. 

This photochemical mode is observed in the case of electron-rich metal centres ligated by poor electron acceptors cyanide complexes, eg [Fen(CN),J4, [Run(CN)6]4 , [Moiv(CN)8]4, and [ WIV(CN)S]4 are known as the most efficient in the eMlv production. A typical reaction is photo-oxidation of hexacyanoferrate(II) ... 

Oxidative addition (electron-rich metal centre) is considered to be the rate limiting step in the catalytic cycle with stericaUy demanding ligands helpful in product formation (reductive elimination). 

The molecular structure of the intermediate alkyl complex, trans-[Rh(CH3)Cl(I)(CO) As(p-Tol)3)2] 3 (Figure 3), clearly shows that trans addition of iodomethane has occurred, as has been found previouslyin systems with sterically congested and electron-rich metal centres. 

By following a related stepwise procedure, aminocarbyne complexes can be used to prepare rare examples of mononuclear bis(aminocarbyne) complexes. Oxidative decarbonylation of Tp W( = CNREt)(CO)2 (R = Me, Et) with Br2 or I2 affords the six-coordinate dihalido-aminocarbyne complexes Tp W( = CNREt) X2 (X = Br, I) in high yield. Subsequent reductive dehalogenation by Na/Hg in the presence of CNEt gives the electron-rich mono-aminocarbyne complexes Tp W( = CNREt)(CNEt)2. An unusually low-field chemical shift for the metal-bonded isonitrile carbons as well as low-frequency Vcn absorptions confirms the presence of a very electron-rich metal centre. Alkylation with [Et30][BF4] at one isocyanide nitrogen yields the bis(aminocarbyne) salts [Tp W( = CNREt) ( = CNEt2)(CNEt)][BF4] (Scheme 23). ... 

Activation of an electrophile by coordination to an electron-poor metal centre (a Lewis acid) and subsequent attack by an external nucleophile is extremely common in organic chemistry. The opposite scenario - the activation of the nucleophile by coordination to an electron-rich metallic centre and electrophilic attack - has been much less pursued. This chemistry can be studied with late transition metals phosphido complexes [LnM-PR2], known to have highly nucleophilic phosphorus atoms. ... 

Electrophilic addition to the sulfur atom of terminal thiocarbonyl ligands is a well-established route to thiocarbyne complexes of Group VI metals [2, 18, 19]. A requirement for the success of this approach is that the thiocarbonyl ligand be bound to a very electron-rich metal centre, preferably in an anionic complex. A methyltellurocarbyne complex of tungsten, [L(CO)2W=CTeMe]... 

These reactions are effected by relatively electron rich metal centres for example, tungstenocene, the probable intermediate in reaction 5 [14], inserts into the weakly activated CH bond of Me4Si but not into a CH bond of neopentane. 

Cp 2MCH3 (M = Sc, Lu, Y) increase with the electrophilicity of the metal [44]. The situation is less clear for oxidative addition. It was long believed that very electron rich metal centres would insert more easily in CH bonds this idea must be revised as we know, for example, that both species 2 and 5 effect oxidative addition. 

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